Laser drive device

ABSTRACT

The laser drive device of this invention includes a laser, first and second current sources, a current amplifier, and first and second transistors. When the first transistor is OFF, a first current from the first current source is supplied to the current amplifier, where the current is amplified to generate a laser current to be supplied to the laser. Thus, the laser is turned ON. During this time, the second transistor is ON, allowing a second current to flow from a power supply node into the second current source. When the first transistor is ON, the entire or part of the first current flows into the second current source through the first transistor. This reduces the current supplied to the current amplifier and thus the laser current, resulting in turning OFF the laser. During this time, the second transistor is OFF. The values of the first and second currents are determined by a set current value. The value of the laser current supplied to the laser during the ON-state of it is determined by the first current. Therefore, by adjusting the set current value, a desired value of laser current can be supplied to the laser. Having the current amplifier, the values of the first and second currents are smaller than the value of the laser current. This suppresses an increase in power consumed by the first and second power sources and the first and second transistors when the laser current increases.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a laser drive device.

[0002] With the recent trend toward larger-capacity and higher-speedoptical disk devices, demands for laser drive devices with high speedand low power consumption have increased for data recording/reproductionfor such optical disk devices.

[0003] As an example of conventional laser drive devices intending toincrease the switching speed, Japanese Patent Publication No. 7-95610discloses a laser drive device as shown in FIG. 9.

[0004] In the conventional laser drive device, an inflow-current I5Aflowing into an inflow-current source 5 and a set current I5B flowingfrom an outflow-current source 6 are set based on a set current I4flowing from a current setting circuit 4. An outflow-current I6 flowingfrom the outflow-current source 6 is set based on the set current I5B.Recording signals reverse to each other are applied to bases oftransistors 3A and 3B of a differential current switch 3. When thetransistor 3A is turned ON and the transistor 3B is turned OFF, acurrent value of a current I3A flowing through the transistor 3A becomesequal to that of the inflow-current I5A and the outflow-current I6. As aresult, a value of a laser current I1 becomes zero, thereby turning OFFa laser 1. When the transistor 3A is turned OFF and the transistor 3B isturned ON, a current value of a current I3B flowing through thetransistor 3B is made equal to that of the inflow-current I5A, while acurrent value of the current I3A flowing through the transistor 3Abecomes zero. As a result, a current value of the laser current I1becomes equal to that of the outflow-current I6, thereby turning ON thelaser 1.

[0005] The above mentioned conventional laser drive device satisfiesdesirable conditions for driving a laser, where the laser 1 is groundedon one side and is connected to the transistor 3A as a switching elementin a collector follower manner on the other side. Moreover, whilesatisfying the above conditions, the transistor 3A as the switchingelement is made of an NPN transistor having a high switching speed. Thisenables easy attainment of a switching speed as high as severalnanoseconds or less.

[0006] The conventional laser drive device shown in FIG. 9 however hasthe following problem.

[0007] In general, in a laser drive device of datarecording/reproduction for an optical disk device, the values of a lasercurrent vary among the operations of reading, erasing, and writing. Thecurrent value is large during writing, while it is small during reading.

[0008] In the illustrated conventional laser drive device, the lasercurrent I1 itself is turned ON/OFF by the differential current switch 3.Therefore, as the laser current I1 is greater, power consumption of thelaser drive device increases.

[0009] As the laser current I1 is smaller, the currents flowing into thetransistors 3A and 3B of the differential current switch 3 decrease,resulting in reducing the switching speed of the transistors 3A and 3B.

SUMMARY OF THE INVENTION

[0010] An object of the present invention is providing a laser drivedevice capable of suppressing increases in power consumption even on anincrease in laser current.

[0011] Another object of the present invention provides a laser drivedevice suppressing decline in switching speed even on decrease in lasercurrent.

[0012] The laser drive device of the present invention includes a laser,a first current source, a second current source, a current amplifier, afirst transistor, and a second transistor. The first current sourcesupplies a first current having a current value associated with a setcurrent value. The second current source receives a second currenthaving a current value associated with the set current value. Thecurrent amplifier amplifies a current from the first current source togenerate a laser current and supplies the laser current to the laser.The first transistor is connected between the first current source andthe second current source. The second transistor is connected between apower supply node receiving a power supply voltage and the secondcurrent source. The first and second transistors are turned ON/OFFcomplementarily.

[0013] In the above laser drive device, when the first transistor isOFF, a first current from the first current source is supplied to thecurrent amplifier. The current amplifier amplifies the current suppliedfrom the first current source to generate a laser current. The lasercurrent is then supplied to the laser. Thus, the laser is turned ON.During this time, the second transistor is ON, allowing a second currentto flow from a power supply node into the second current source throughthe second transistor. When the first transistor is ON, the entire orpart of the first current flows into the second current source throughthe first transistor. This reduces the current supplied to the currentamplifier and thus reduces the laser current, resulting in turning OFFthe laser. During this time, the second transistor is OFF. The values ofthe first and second currents are determined by a set current value. Thevalue of the laser current supplied to the laser during the ON-state ofit is determined by the value of the first current. Therefore, byadjusting the set current value, a desired value of laser current can besupplied to the laser.

[0014] Since the laser drive device is provided with the currentamplifier, the values of the first and second currents are smaller thanthe value of the laser current. This suppresses an increase in powerconsumed by the first and second power sources and the first and secondtransistors.

[0015] Preferably, the anode of the laser is connected to the powersupply node, and the current amplifier includes first, second, and thirdNPN transistors. The first NPN transistor is connected between the firstcurrent source and a grounding node receiving a grounding potential withthe emitter being grounded. The second NPN transistor has a collectorconnected to the power supply node, an emitter connected to the base ofthe first NPN transistor, and a base connected to the collector of thefirst NPN transistor. The third NPN transistor is connected between thecathode of the laser and the grounding node with the emitter beinggrounded. The base of it is connected to the base of the first NPNtransistor.

[0016] Preferably, the current amplifier further includes a plurality offourth NPN transistors connected between the cathode of the laser andthe grounding node in parallel with the third NPN transistor with theemitters being grounded. The bases of the fourth NPN transistors areconnected to the base of the first NPN transistor.

[0017] In the above laser drive device, the first, second, and third NPNtransistors make a current mirror circuit. The current from the firstcurrent source flows through the first NPN transistor. A current of avalue obtained by multiplying the current flowing through the first NPNtransistor by the mirror ratio flows through the third NPN transistor,to be supplied to the laser as the laser current.

[0018] Further, the first and second NPN transistors and each of theplurality of fourth NPN transistors make a current mirror circuit. Thesum of the currents flowing through the respective fourth NPNtransistors and the current flowing through the third NPN transistor issupplied to the laser as the laser current.

[0019] Preferably, the anode of the laser is connected to the powersupply node, and the current amplifier includes first, second, and thirdn-channel MOS transistors. The first n-channel MOS transistor isconnected between the first current source and a grounding nodereceiving grounding potential. The second n-channel MOS transistor isconnected between the power supply node and the gate of the firstn-channel MOS transistor. The gate of the second n-channel MOStransistor is connected to the first current source. The third n-cannelMOS transistor is connected between the cathode of the laser and thegrounding node. The gate of the third n-channel MOS transistor isconnected to the gate of the first n-channel MOS transistor.

[0020] Preferably, the current amplifier further includes a plurality offourth n-channel MOS transistors connected between the cathode of thelaser and the grounding node in parallel with the third n-channel MOStransistor. The gates of the fourth n-channel MOS transistors areconnected to the gate of the first n-channel MOS transistor.

[0021] In the above laser drive device, the first, second, and thirdn-channel MOS transistors make a current mirror circuit. The currentfrom the first current source flows through the first NPN transistor. Acurrent of a value obtained by multiplying the current flowing throughthe first NPN transistor by the mirror ratio flows through the third NPNtransistor, to be supplied to the laser as the laser current.

[0022] Further, the first and second n-channel MOS transistors and eachof the plurality of fourth n-channel MOS transistors make a currentmirror circuit. The sum of the currents flowing through the respectivefourth n-channel MOS transistors and the current flowing through thethird n-channel MOS transistor is supplied to the laser as the lasercurrent.

[0023] Preferably, the above laser drive device further includes a thirdcurrent source connected to a node interconnecting the first transistorand the second current source for receiving a third current.

[0024] In the above laser drive device, when the first transistor isOFF, the second transistor is ON, allowing the sum of the second andthird currents to flow through the second transistor. The second currentflows into the second current source while the third current flows intothe third current source. When the first transistor is ON, the secondtransistor is OFF, allowing the entire or part of the first current fromthe first current source to flow through the first transistor. Thiscurrent is equal to the sum of the second and third currents, where thesecond current flows into the second current source while the thirdcurrent flows into the third current source.

[0025] Overall, as the value of a current flowing through a transistoris smaller, the switching speed of the transistor is lower. In the abovelaser drive device, as the value of the laser current supplied to thelaser is smaller, the value of the second current decreases. However,since the above laser drive device is provided with the third currentsource, the constant third current flows into one of the first andsecond transistors even when the value of the laser current is small.Thus,, the switching speed of the first and second transistors issuppressed from decreasing even when the laser current is small.

[0026] Preferably, the above laser drive device further includes a firstdiode and a voltage application means. The first diode has an anodeconnected to the first current source and a cathode connected to thecurrent amplifier. The voltage application means applies a predeterminedvoltage in the forward direction with respect to the first diode.

[0027] In the above laser drive device, with the placement of the firstdiode, reverse current flow from the current amplifier is prevented.

[0028] However, an intense reverse bias may undesirably be applied tothe first diode. To avoid this occurrence in the laser drive device, itprovides the voltage application means to apply a voltage on the firstdiode that is too low to turn ON in the forward direction with respectto the first diode.

[0029] The above form provides the following additional effect.

[0030] When the laser is OFF, that is, the first transistor is ON, thevoltage at the node interconnecting the first transistor and the firstcurrent source decreases. This decrease is however only to the level ofa voltage applied by the voltage application means. Therefore, when thefirst transistor is turned OFF next, the time required for the voltageat the interconnecting node to reach a predetermined level is shortened,compared with the case of having no voltage application means. That is,the switching speed can be made higher.

[0031] It is preferable that the voltage application means includes afourth current source, m pieces of second diode, and a third transistor.The fourth current source supplies a fourth current. The m pieces ofsecond diode are connected in series between the fourth current sourceand the cathode of the first diode. The third transistor has a collectorconnected to the power supply node, an emitter connected to the anode ofthe first diode, and a base connected to the fourth current source.

[0032] In the above laser drive device, a difference voltage between adropped voltage at the m pieces of second diode due to the fourthcurrent and a base-emitter voltage at the third transistor is applied inthe forward direction to the first diode.

[0033] Preferably, the voltage application means further includes npieces of third diode connected in series between the emitter of thethird transistor and the anode of the first diode.

[0034] In the above laser drive device, the voltage that is applied inthe forward direction to the first diode is lower by a dropped voltageat the n pieces of third diode than the difference voltage between adropped voltage at the m pieces of second diode due to the fourthcurrent and a base-emitter voltage at the third transistor. Therefore,the first diode can be turned OFF without fail even in the case wherethe first diode fails to be sufficiently turned OFF with the voltageapplied thereto using only the m second diodes and the third transistor.

[0035] Preferably, the laser drive device further includes a fifthcurrent source connected to the cathode of the first diode for receivinga fifth current.

[0036] If the fifth current source is not provided, the fourth currentmay be supplied to the current amplifier when the laser is OFF, that is,the first transistor is ON, possibly resulting in turning ON the laser.

[0037] In the above laser drive device, the entire fourth current orpart of it flows into the fifth current source as the fifth current.Therefore, the above mentioned inconvenience can be avoided.

[0038] Preferably, the voltage application means further includes aresistor connected between the emitter of the third transistor and theanode of the first diode.

[0039] In the above laser drive device, a voltage is applied to theforward direction of the first diode that is lower by a value of adropped voltage at the resistor than the difference voltage between adropped voltage at the m pieces of second diode due to the fourthcurrent and a base-emitter voltage at the third transistor. Therefore,the first diode can be turned OFF without fail even in the case wherethe first diode fails to be sufficiently turned OFF with the voltageapplied thereto using only the m second diodes and the third transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040]FIG. 1 is a view illustrating the overall form of a laser drivedevice of EMBODIMENT 1 of the present invention.

[0041]FIG. 2 is a timing chart for description of the operation of thelaser drive device of FIG. 1.

[0042]FIG. 3 is a view illustrating an alternation of the laser drivedevice of FIG. 1.

[0043]FIG. 4 is a view illustrating the overall form of a laser drivedevice of EMBODIMENT 2 of the present invention.

[0044]FIG. 5 is a timing chart for description of the operation of thelaser drive device of FIG. 4.

[0045]FIG. 6 is a view illustrating the overall form of a laser drivedevice of EMBODIMENT 3 of the present invention.

[0046]FIG. 7 is a view illustrating the overall form of a laser drivedevice of EMBODIMENT 4 of the present invention.

[0047]FIG. 8 is a view illustrating the overall form of a laser drivedevice of EMBODIMENT 5 of the present invention.

[0048]FIG. 9 is a view illustrating the overall form of a conventionallaser drive device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0049] Hereinafter, preferred embodiments of the present invention willbe described with reference to the accompanying drawings. In thedrawings, the same or corresponding components are denoted by the samereference numerals, and the description on these components is notrepeated.

[0050] Embodiment 1

[0051]FIG. 1 illustrates the overall form of the laser drive device ofEMBODIMENT 1 according to the present invention. Referring to FIG. 1,the laser drive device of this embodiment includes a current settingcircuit 10, an inflow-current source 11, an outflow-current source 12, adifferential current switch 13, a laser 14, a current amplifier 20, anda constant current source 30.

[0052] The current setting circuit 10 generates a desired set currentI11.

[0053] The inflow-current source 11 includes NPN transistors 11A through11C and resistors 11D through 11F. The NPN transistor 11A has acollector connected to the output of the current setting circuit 10, anemitter connected to the resistor 11D, and a base connected to thecollector of it. The NPN transistor 11B has a collector connected to anode N1, an emitter connected to the resistor 11E, and a base connectedto the base of the NPN transistor 11A. The NPN transistor 11C has acollector connected to the outflow-current source 12, an emitterconnected to the resistor 11F, and a base connected to the base of theNPN transistor 11A. The resistors 11D through 11F are connected betweenthe emitters of the NPN transistors 11A through 11C, respectively, and agrounding node GND.

[0054] The outflow-current source 12 includes PNP transistors 12A and12B and resistors 12C and 12D. The PNP transistor 12A has an emitterconnected to the resistor 12C, a collector connected to a node N2, and abase connected to the base and collector of the PNP transistor 12B. ThePNP transistor 12B has an emitter connected to the resistor 12D, acollector connected to the collector of the NPN transistor 11C of theinflow-current source 11, and a base connected to the collector of it.

[0055] The differential current switch 13 includes NPN transistors 13Aand 13B. The NPN transistor 13A has a collector connected to the nodeN2, an emitter connected to the node N1, and a base receiving a signalSD1. The NPN transistor 13B has a collector connected to a power supplynode Vcc, an emitter connected to the node N1, and a base receiving asignal SD2.

[0056] The current amplifier 20 includes NPN transistors 21A through 21Dand resistors 22A through 22C. The NPN transistor 21A has a collectorconnected to the power supply node Vcc, an emitter connected to the baseof the NPN transistor 21B, and a base connected to the collector of theNPN transistor 21B. The NPN transistor 21B has a collector connected tothe node N2, an emitter connected to the resistor 22A, and a baseconnected to the emitter of the NPN transistor 21A. The NPN transistors21C and 21D have respective collectors connected to a node N3, emittersconnected to the resistors 22B and 22C, respectively, and respectivebases connected to the base of the NPN transistor 21B. The resistors 22Athrough 22C are connected between the emitters of the NPN transistors21B through 21D, respectively, and the grounding node GND.

[0057] The laser 14 has an anode connected to the power supply node Vccand a cathode connected to the node N3.

[0058] The constant current source 30 is provided between the node N1and the grounding node GND for allowing a current Ic to flow from thenode N1 toward the grounding node GND.

[0059] The operation of the laser drive device with the above form willbe described.

[0060]FIG. 2 is a timing chart for describing the operation of the laserdrive device shown in FIG. 1.

[0061] Referring to FIGS. 1 and 2, the set current I11 from the currentsetting circuit 10 is supplied to the collector and base of the NPNtransistor 11A of the inflow-current source 11. The NPN transistors 11Aand 11B make a current mirror circuit, so that an inflow-current Ib isdetermined by the set current I11 and the mirror ratio of the currentmirror circuit. The NPN transistors 11A and 11C also make a currentmirror circuit, so that an inflow-current I13 is determined by the setcurrent I11 and the mirror ratio of this current mirror circuit.

[0062] The PNP transistors 12A and 12B of the outflow-current source 12make a current mirror circuit, so that an outflow-current Ia isdetermined by the inflow-current I13 and the mirror ratio of thiscurrent mirror circuit. This means that the outflow-current Ia isdetermined by the set current I11. In this embodiment, it is assumedthat the mirror ratios of the respective current mirror circuits are setso that the value of the outflow-current Ia is equal to the sum of theinflow-current Ib and the current Ic.

[0063] Signals SD1 and SD2 complementary to each other are applied tothe bases of the transistors 13A and 13B, respectively, of thedifferential current switch 13.

[0064] When the laser 14 is to be turned ON, the signal SD1 is set to L(low) level and the signal SD2 is set to H (high) level. Here, the NPNtransistor 13A is set to turn OFF and the NPN transistor 13B is set toturn ON, whereby the value of the current I15B flowing through the NPNtransistor 13B is made equal to the sum of the value of theinflow-current Ib and the current Ic, while a current I15A flowingthrough the NPN transistor 13A is zero. As a result, a current I10supplied to the current amplifier 20 is equal to the outflow-current Ia.In the current amplifier 20, the NPN transistors 21A through 21C make acurrent mirror circuit. Therefore, the current I10 flows_through the NPNtransistor 21B, and a current I20A of a value obtained by multiplyingthe current I10 by the mirror ratio of this current mirror circuit isinduced at the NPN transistor 21C. The NPN transistors 21A, 21B, and 21Dalso make a current mirror circuit. Therefore, a current I20B of a valueobtained by multiplying the current I10 by the mirror ratio of thiscurrent mirror circuit is induced at the NPN transistor 21D. The sum ofthe currents I20A and I20B is equal to a laser current I17 supplied tothe laser 14. Thus, the laser 14 is turned ON. The relationship betweenthe currents I10 and I17 is expressed by I17=I10×Ai in which Ai denotesthe gain of the current amplifier 20.

[0065] When the laser 14 is to be turned OFF, the signal SD1 is turnedto H level while the signal SD2 is turned to L level. This turns ON theNPN transistor 13A and turns OFF the NPN transistor 13B. Accordingly,the entire outflow-current Ia becomes the current I15A flowing throughthe NPN transistor 13A, and further becomes the inflow-current Ib andthe current Ic. As a result, the current I10 supplied to the currentamplifier 20 becomes zero, and thus the laser 14 is turned OFF.

[0066] When the laser drive device operating as described above isapplied to an optical disk device, the values of the laser current I17vary for writing, reading, and erasing of data on/from an optical disk.Overall, the value of the laser current I17 is large during writing,while it is small during reading. In the laser drive device of thisembodiment, the values of the inflow-current Ib and the outflow-currentIa are adjusted by adjusting the value of the set current I11. Further,by the adjustment of the outflow-current Ia, the value of the lasercurrent I17 is adjusted. In other words, a required value of the lasercurrent I17 can be supplied to the laser 14 by adjusting the value ofthe set current I11 supplied from the current setting circuit 10.

[0067] In the laser drive device of this embodiment, which is providedwith the current amplifier 20, the values of the outflow-current Ia andthe inflow-current Ib are smaller than the value of the laser currentI17. This makes it possible to suppress an increase in power consumed bythe current setting circuit 10, the inflow-current source 11, theoutflow-current source 12, and the differential current switch 13 evenwhen the laser current I17 supplied to the laser 14 increases duringwriting of data on an optical disk, for example.

[0068] Overall, as a current flowing through a transistor is smaller,the switching speed of the transistor is lower. In the laser drivedevice of this embodiment, as the value of the laser current I17supplied to the laser 14 is smaller, the value of the inflow-current Ibis smaller. However, since the laser drive device of this embodiment isprovided with the constant current source 30, the constant current Icflows through one of the NPN transistors 13A and 13B even when the valueof the laser current I17 is small. Thus, the switching speed of the NPNtransistors 13A and 13B can be suppressed from decreasing even when thelaser current I17 supplied to the laser 14 is small.

[0069] In the above description, the value of the outflow-current Ia wasset to be equal to the sum of the inflow-current Ib and the current Ic.Alternatively, part of the outflow-current Ia may be set to be equal tothe sum of the inflow-current Ib and the current Ic. In this case,however, it is required that the value of the laser current I17determined by the part of the outflow-current Ia supplied to the currentamplifier 20 as the current I10 is smaller than a threshold value of thelaser 14.

[0070] Alternatively, a plurality of additional NPN transistors andresistors may be provided between a node N3 and the grounding node GNDin parallel with the NPN transistors 21C and 21D and the resistors 22Band 22C. With this form, the gain of the current amplifier 20 can beadjusted to a desired value.

[0071]FIG. 3 illustrates another laser drive device of this embodiment,where n-channel and p-channel MOS transistors are used in place of theNPN and PNP transistors. Specifically, n-channel MOS transistors 11Athrough 11C may be used in place of the NPN transistors 11A through 11Cof the inflow-current source 11, p-channel MOS transistors 12A and 12Bin place of the PNP transistors 12A and 12B of the outflow-currentsource 12, n-channel MOS transistors 13A and 13B in place of the NPNtransistors 13A and 13B of the differential current switch 13, andn-channel MOS transistors 21A through 21D in place of the NPNtransistors 21A through 21D of the current amplifier 20.

[0072] Embodiment 2

[0073]FIG. 4 illustrates the overall form of the laser drive device ofEMBODIMENT 2 according to the present invention. The laser drive deviceof this embodiment shown in FIG. 4 includes, in addition to thecomponents of the laser drive device shown in FIG. 1, a reverse flowprevention diode 40, an outflow-current source 50, a constant currentsource 60, clamping diodes 71 and 72, and a clamping transistor 73. Theoutflow-current source 50, the constant current source 60, the clampingdiodes 71 and 72, and the clamping transistor 73 make a voltageapplication means.

[0074] The reverse flow prevention diode 40 is an NPN transistor ofwhich collector and base are connected with each other, where thecollector and the base make an anode while the emitter makes a cathode.The anode is connected to the node N2, while the cathode is connected tothe collector of the NPN transistor 21B of the current amplifier 20.

[0075] The outflow-current source 50 includes PNP transistors 50A and50B and resistors 50C and 50D. The PNP transistor 50A has an emitterconnected to the resistor 50C, a collector connected to a node N4, and abase connected to the base and collector of the PNP transistor 50B. ThePNP transistor 50B has an emitter connected to the resistor 50D, acollector connected to the constant current source 60, and a baseconnected to the collector of it. The resistors 50C and 50D areconnected between the power supply node Vcc and the emitters of the PNPtransistors 50A and 50B, respectively.

[0076] The constant current source 60 is provided between the collectorof the PNP transistor 50B and the grounding node GND for allowing acurrent I20 to flow from the collector of the PNP transistor 50B towardthe grounding node GND.

[0077] Each of the clamping diodes 71 and 72 is an NPN transistor ofwhich collector and base are connected with each other, where thecollector and the base make an anode while the emitter makes a cathode.The anode of the clamping diode 71 is connected to the node N4, whilethe cathode of it is connected to the anode of the clamping diode 72.The cathode of the clamping diode 72 is connected to the cathode of thereverse flow prevention diode 40.

[0078] The clamping transistor 73 has a collector connected to the powersupply node Vcc, an emitter connected to the anode of the reverse flowprevention diode 40, and a base connected to the node N4.

[0079] The operation of the laser drive device with the above form willbe described.

[0080]FIG. 5 is a timing chart for describing the operation of the laserdrive device shown in FIG. 4.

[0081] Referring to FIGS. 4 and 5, when the laser 14 is to be turned ON,the signal SD1 is turned to L level while the signal SD2 is turned to Hlevel. This turns OFF the NPN transistor 13A and turns ON the NPNtransistor 13B, whereby the current I15B flowing through the NPNtransistor 13B is made equal to the sum of the inflow-current Ib and thecurrent Ic, while the current I15A flowing through the NPN transistor13A becomes zero. As a result, the current I10 supplied to the currentamplifier 20 is equal to the sum of the outflow-current Ia, a currentI21, and an outflow-current Id. Thus, the laser 14 is turned ON.

[0082] When the laser 14 is to be turned OFF, the signal SD1 is turnedto H level while the signal SD2 is turned to L level. This turns ON theNPN transistor 13A and turns OFF the NPN transistor 13B, by which theoutflow-current Ia and the current I21 become the current I15A flowingthrough the NPN transistor 13A, and further become the inflow-current Iband the current Ic. As a result, the current I10 supplied to the currentamplifier 20 is equal to the outflow-current Id, which is too low in alevel to influence the ON/OFF of the laser 14. Thus, the laser 14 isturned OFF.

[0083] Since the laser drive device of this embodiment is provided withthe reverse flow prevention diode 40, reverse current flow from thecurrent amplifier 20 toward the node N2 is prevented. However, anintense reverse bias may undesirably be applied to the reverse flowprevention diode 40. To avoid this occurrence, the voltage applicationmeans is provided.

[0084] The NPN transistors 50A and 50B of the outflow-current source 50make a current mirror circuit, so that the outflow-current Id that flowsthrough the PNP transistor 50A is determined by the constant current I20and the mirror ratio of this current mirror circuit. The voltage levelat the node N4 is higher than that at the cathode of the reverse flowprevention diode 40 by a value corresponding to the dropped voltage atthe clamping diodes 71 and 72 caused by the outflow-current Id. Thevoltage at the node N4 is applied to the base of the clamping transistor73, to allow the current I21 to flow. Therefore, the voltage level atthe anode of the reverse flow prevention diode 40 is lower than that atthe node N4 by a value corresponding to the base-emitter voltage at theclamping transistor 73. In this way, a voltage that is too low to turnON the reverse flow prevention diode 40 is applied in the forwarddirection with respect to the reverse flow prevention diode 40.

[0085] The laser drive device of this embodiment with the above formprovides the following additional effect.

[0086] When the laser 14 is to be turned OFF, the NPN transistor 13A isturned ON, by which the outflow-current Ia and the current I21 becomethe current I15A flowing through the NPN transistor 13A, and furtherbecome the inflow-current Ib and the current Ic. This reduces thevoltage level at the node N2. This reduction is however only to thelevel equal to that of the voltage at the anode of the reverse flowprevention diode 40 applied by the voltage application means. Therefore,when the NPN transistor 13A is turned OFF next, the time required forthe voltage level at the node N2 to reach a predetermined level isshortened, compared with the case of having no voltage applicationmeans. That is, the switching speed of the NPN transistors 13A and 13Bis made higher.

[0087] In this embodiment, two clamping diodes, 71 and 72, were used.The number of the clamping diodes is not limited, but any number ofclamping diodes required for application of a voltage too low to turn ONthe reverse flow prevention diode 40 can be provided.

[0088] Embodiment 3

[0089]FIG. 6 illustrates the overall form of the laser drive device ofEMBODIMENT 3 according to the present invention. The laser drive deviceof this embodiment shown in FIG. 6 includes a constant current source80, in addition to the components of the laser drive device shown inFIG. 4. The constant current source 80 is provided between the cathodeof the reverse flow prevention diode 40 and the grounding node GND forallowing a constant current Ie to flow from the cathode of the reverseflow prevention diode 40 toward the grounding node GND.

[0090] In the laser drive device shown in FIG. 4, the outflow-current Idis supplied to the current amplifier 20 when the laser 14 is to beturned OFF. If the value of the laser current I17 obtained by amplifyingthe outflow-current Id exceeds the threshold value of the laser 14, thelaser 14 will be turned ON.

[0091] In the laser drive device of this embodiment shown in FIG. 6, theoutflow-current Id is drawn by the constant current Ie, so that thelaser 14 is prevented from being turned ON when the laser 14 should bein the OFF state.

[0092] Embodiment 4

[0093]FIG. 7 illustrates the overall form of the laser drive device ofEMBODIMENT 4 according to the present invention. The laser drive deviceof this embodiment shown in FIG. 7 includes a resistor 90, in additionto the components of the laser drive device shown in FIG. 4. Theresistor 90 is connected between the emitter of the clamping transistor73 and the anode of the reverse flow prevention diode 40.

[0094] In the laser drive device shown in FIG. 4, the voltage level atthe anode of the reverse flow prevention diode 40 may not be loweredsufficiently, thereby failing to turn off the reverse flow preventiondiode 40 sufficiently. In such an occurrence, the current I21 maypresumably be partly supplied to the current amplifier 20 resulting inturning ON the laser 14.

[0095] In the laser drive device of this embodiment shown in FIG. 7, thevoltage level at the anode of the reverse flow prevention diode 40 canbe reduced by a value corresponding to a dropped voltage caused by theresistor 90. As a result, the reverse flow prevention diode 40 can beturned OFF without fail.

[0096] In this embodiment, the resistor 90 was provided. Alternatively,a diode may be provided between the emitter of the clamping transistor73 and the anode of the reverse flow prevention diode 40.

[0097] In place of the resistor 90, also, a plurality of diodesconnected in series may be provided between the emitter of the clampingtransistor 73 and the anode of the reverse flow prevention diode 40.

[0098] Embodiment 5

[0099]FIG. 8 illustrates the overall form of the laser drive device ofEMBODIMENT 5 according to the present invention. The laser drive deviceof this embodiment shown in FIG. 8 includes, in addition to thecomponents of the laser drive device shown in FIG. 1, the reverse flowprevention diode 40, the outflow-current source 50, the constant currentsource 60, the clamping diodes 71 and 72, and the clamping transistor 73shown in FIG. 4, the constant current source 80 shown in FIG. 6, and theresistor 90 shown in FIG. 7.

[0100] In the laser drive device of this embodiment, when the laser 14is to be turned ON, the signal SD1 is turned to L level while the signalSD2 is turned to H level. This turns OFF the NPN transistor 13A andturns ON the NPN transistor 13B, whereby the current I15B flowingthrough the NPN transistor 13B is made equal to the sum of theinflow-current Ib and the current Ic, while the current I15A flowingthrough the NPN transistor 13A becomes zero. As a result, the currentI10 supplied to the current amplifier 20 is equal to the sum of theoutflow-current Ia, the current I21, the outflow-current Id, and theinflow-current Ie. The laser current I17 is generated by the voltageamplifier 20 based on the current I10, and supplied to the laser 14 toturn ON the laser 14.

[0101] When the laser 14 is to be turned OFF, the signal SD1 is turnedto H level while the signal SD2 is turned to L level. This turns ON theNPN transistor 13A and turns OFF the NPN transistor 13B, by which theoutflow-current Ia and the current 121 become the current I15A flowingthrough the NPN transistor 13A, and further become the inflow-current Iband the current Ic. The current Id is drawn by the constant current Ie.Thus, the laser 14 is turned OFF.

[0102] In the laser drive device of this embodiment, since the currentamplifier 20 is provided, the values of the outflow-current Ia and theinflow-current Ib are smaller than the value of the laser current I17.This makes it possible to suppress an increase in power consumed by thecurrent setting circuit 10, the inflow-current source 11, theoutflow-current source 12, and the differential current switch 13 evenwhen the laser current I17 supplied to the laser 14 increases duringwriting of data on an optical disk, for example.

[0103] Since the constant current source 30 is provided, the constantcurrent Ic flows through the NPN transistors 13A and 13B even when thevalue of the laser current I17 is small. This makes it possible tosuppress decrease in the switching speed of the NPN transistors 13A and13B even when the laser current I17 supplied to the laser 14 is small.

[0104] Since the voltage application means is provided, the reverse flowprevention diode 40 is prevented from receiving an intense reverse bias.In addition, the switching speed of the NPN transistors 13A and 13B ismade higher compared with the case of providing no voltage applicationmeans.

[0105] Since the constant current source 80 is provided, the laser 14 isprevented from being turned ON when it should be in the OFF state.

[0106] Since the resistor 90 is provided, the OFF state of the reverseflow prevention diode 40 is ensured.

[0107] It should be note that the laser drive devices of EMBODIMENTS 1through 5 described above are operable if the connection polarity at thelaser 14 is changed and the PNP transistors and the NPN transistors areexchanged with each other.

[0108] While the present invention has been described in a preferredembodiment, it will be apparent to those skilled in the art that thedisclosed invention may be modified in many ways and may assume manyembodiments other than that specifically set out and described above.Accordingly, it is intended by the appended claims to cover allmodifications of the invention which fall within the true spirit andscope of the invention.

What is claimed is:
 1. A laser drive device comprising: a terminal beingconnected to a laser; a first current source for supplying a firstcurrent having a current value associated with a set current value; asecond current source for receiving a second current having a currentvalue associated with the set current value; a current amplifier foramplifying a current from the first current source to generate a lasercurrent and supplying the laser current to the laser; a first electricalswitch connected between the first current source and the second currentsource; and a second electrical switch connected between a power supplynode receiving a power supply voltage and the second current source, thesecond electrical switch being turned ON/OFF complementarily to thefirst electrical switch.
 2. A laser drive device of claim 1, whereinsaid electrical switch is a transistor.
 3. The laser drive device ofclaim 1, further comprising a third current source being connected tothe connected portion of said first and second electrical switch withthe second current source.